Most of the shazam remakes were made from scratch. If you compare something like shazam33 to shazam, you'll notice none of the original brushwork is there. Decompiling maps is really bad practice. If the mapper hasn't provided the .map source, then he/she probably doesn't want people editing it without permission. GPL-ing maps is a pretty recent practice, so most maps aren't released under the GPL.
It's best to just be original. Make something new. Something better than previous maps.